I. Field of the Invention
This invention relates to an electrical fuse assembly having an optical indicator to warn of a blown fuse.
II. Description of the Prior Art
West German Offenlegungsschrift No. 31 03 478 discloses a safety fuse device having an insulating body and a fusible conductor arranged within the body between two contacts, the fusible conductor being embedded in a quenching medium. A warning circuit comprising a high series resistance glow lamp in parallel to the fusible conductor is provided, wherein the glow lamps serves as a clearly visible indicator of the condition of the fuse device. Such a system is intended to replace conventional mechanical warning devices in electrical safety fuse assemblies.
Instead of the glow lamp, a suitable electro-optical indicator device, for example, an incandescent filament light, a light-emitting diode, or a liquid crystal indicator, may be provided, and is installed in a condition indicator circuit parallel to the fusible conductor.
West German Offenlegungsschrift No. 27 41 779 discloses an electrical fuse which consists of a transparent envelope, a fusible conductor located within the envelope and a switching circuit connected to the fusible conductor and connectable with a power supply. An electro-optical warning device is formed by a light-emitting diode which is connected in series to a resistor, whereby the light-emitting diode is located within the envelope and is connected to the resistor and the switching circuit in such a way that both the light-emitting diode and the resistor are located parallel to the fusible conductor. A heat insulating body within the envelope protects the light-emitting diode from the heat generated when the fusible conductor is burnt through. When this occurs, current passes through the resistor and the light-emitting diode to indicate a burnt out electrical fuse.
West German Offenlegungsschrift No. 25 04 582 discloses a re-usable fuse cartridge in the form of a tube having a glow lamp located therein. The glow lamp is connected in series to a high resistance. Several fusible conductors are located on the outside of the tube. An additional special fuse holder is necessary for this type of fuse cartridge.
Known fuse assemblies which have electro-optical condition indicators, because of the additional elements necessary for the condition indicator, including discrete series resistances, have a small internal volume that is usable for quenching the breakdown arc which is generated when the fusible connector is burnt through. Consequently, the fuse capacity or electrical load which can be accommodated is very limited. The testable fuse capability of the known fuses do not comply with those specified for D- and NH-fuse systems according to VDE 0635, VDE 0636, DIN 57635 and DIN 57636 and for the granting of VDE-test symbols. Therefore, known safety fuse assemblies incorporating optical display devices are not of much practical use.
In addition to these deficiencies, it should also be noted that if screw caps having integrated light-emitting condition indicators are used to indicate the condition of a fuse assembly, then either a manually operable test contact for testing the fuse condition or a long contact connector between the fuse assembly and a gauge ring to the fuse foot, is required. Both arrangements are complicated in construction and difficult to operate by the user.